1. Field of the Invention
The present invention is concerned with an improved, modular screw anchor (and a method of installing the same) which is characterized by extremely high resistance to breakage, particularly during installation when the anchor is subjected to high torsional loadings. More particularly, it is concerned with such modular screw anchors having separate rod and anchor members and which are designed to be installed using conventional, existing screw anchor wrenches and the like. A prime feature of the anchors in accordance with the invention is that greatly improved resistance to breakage is obtained despite the fact that the quantity of metal used in fabrication of the anchors is essentially the same as that of prior units having significantly lower resistance to breakage.
2. Description of the Prior Art
Present day earth screw anchors used by utilities and others for guying purposes generally comprise an elongated, solid, square in cross-section shaft having one or more helical, outwardly extending, load-bearing blades welded thereto, and an elongated, upwardly extending anchor rod threadably coupled to the anchor shaft. In installation procedures, an elongated anchor wrench including a tubular shank is telescoped over the anchor rod and drivingly engages the anchor shaft at the region of the helical blade or blades. Such engagement is established by virtue of a mating fit between the square cross-section of the shaft, and the corresponding square tubular configuration of the wrench shank. Installation of the anchor is accomplished by powered rotation of the wrench which in turn effects corresponding rotation of the anchor shaft and blade, so that the anchor is screwed into the earth. This is continued until the blade reaches sub-soil having a density sufficient for holding purposes.
Although conventional anchors and installation methods are well established, a number of problems remain. One of the most serious difficulties of relatively recent origin stems from the fact that the torque capacity of modernday installation equipment significantly exceeds the maximum torsional strengths of standard anchors. That is to say, newer installation equipment will commonly have a torque capacity in the range of ten thousand foot-pounds or more, whereas standard anchors have rated strengths in the range of four to seven thousand foot-pounds. As a result, utilities have experienced anchor breakage during installation, particularly in hard, dense, rocky soils. The principal breakage mode is that of anchor shaft breakage under torsional load when the helix strikes an obstruction. In other cases the helix may be stripped from the shaft, or the anchor wrench may split and simply rotate relative to the earth anchor.
One possible solution to these problems is to simply use thicker and stronger metal components in the anchor shaft and helix. However, this alternative is not a practical solution because of the cost involved and more importantly because it would necessitate the purchase of new anchor wrenches and related equipment. Obviously, utilities are loathe to simply scrap their expensive installation equipment if another solution is available. Accordingly, attempts have been made to strengthen conventionally sized earth anchors by improving material quality (e.g., by using special steel alloys having enhanced torque strength) and/or heat treating the central anchor shafts. However, these efforts have achieved only limited success, and are also relatively (twice or more) expensive.
Another fact of present anchors is the loss in strength which is inherent in the factory welding of the helix to the central shaft. This weld loss typically ranges from a few percent to as high as 10 to 15% and is directly subtractive from the strength of the central shaft. This can lead to field breakage during installation attempts by utilities and is a source of increased costs and constant concern on the part of the user. For the reliability required in utility electric distribution lines, the user must be confident that the anchor as installed will be undamaged after it has been subjected to installation torques. With present systems the user is forced to select the product torques just adequate for installation because of product costs and the wrench system in use, and this increases the risk of undetected damage to the anchor during installation.
Another limitation of present screw anchors is that when a strong anchor installation is desired in a particular soil, the maximum rod size, which must always be smaller in cross-section than the anchor shaft, limits the load holding capacity. This is true because the only practical way to attach the anchor rod to the anchor shaft is by drilling and tapping the shaft. The threaded connection zone thus becomes the holding power limit. Present maximums are about 36,000 lbs., obtained with a 1" rod.
Yet another problem associated with conventional anchors stems from the integral welded construction of the shaft-helix combination. Because of this, users are required to stock a wide variety of anchor sizes and types ( e.g., single or multiplex helix units) to meet the varying demands encountered in the field. As such, problems or ordering, warehousing and cost are multiplied.